Piezoelectric devices such as actuators have been the subject of much attention in recent years, due to the promise they hold for improved precision, robustness and reliability in various applications. In the art of fuel injectors, piezoelectric actuators are commonly coupled with a control valve to control the timing, duration or rate shape of fuel injection events. In general terms, a piezoelectric actuator will include one or more piezoelectric elements which, when subjected to an electrical potential, experience a conformational change. This phenomenon is leveraged to relatively precisely control the position of a component of interest coupled with the actuator, in the case of a fuel injector a control valve as mentioned above. Despite the heightened interest in piezoelectric actuators in recent years, they have yet to achieve widespread commercial use in fuel systems. Issues relating to manufacturing, assembly and operation of piezoelectric elements used in such actuators continue to challenge fuel system manufacturers.
One problem with piezoelectric actuators relates to setting the “preload” on the piezoelectric element used therein. As is well understood by those familiar with piezoelectric actuators, the piezoelectric element must typically be held in compression for it to respond predictably and reliably to an applied electrical potential. Many proposals for piezoelectric actuators couple the actuator with a relatively small, low-flow control valve which is moved rapidly to control pressure and/or flow of a larger volume of fuel within a fuel injector. In such instances, the need for predictability and reliability will be readily apparent. Where preload on the piezoelectric element of the actuator is too high or too low, however, the piezoelectric element may not behave as desired.
In a related vein, many earlier strategies for piezoelectric actuator design and assembly were driven largely by preloading concerns. In other words, traditional manufacturing goals such as reducing the number of parts, the complexity of components or eliminating assembly steps have heretofore been a relatively low priority for many designers. As a result, state of the art manufacturing strategies tend to be relatively complicated, and piezoelectric actuators are in many cases relatively cost-inefficient to make. One known piezoelectric actuator device having a relatively small number of parts is set forth in U.S. Pat. No. 7,145,282 B2 to Oakley et al. In designs proposed by Oakley et al., preloading of a stack of piezoelectric disks is purportedly achieved via elasticity of a housing for the actuator. This would appear to offer the advantage of not needing a separate element to apply the preload, as the preloading element is integrated into the housing. One disadvantage, however, is that precise preloading with Oakley et al. may be relatively difficult.